To all;
An aluminum grounding plate is not such a bad idea, provided that you deal
with it properly.
First, the simplest is the use of an Internal Tooth Star Washer between the
lug and the strip. No, not just under the screw head or nut but between the
connecting pieces! The ones with the teeth pointing towards the screw's
shaft. When tightened firmly, the washer's teeth will dig into the surfaces
of the joined pieces of metal to a depth, gouging through the oxide layer to
produce a metal-to-metal joint which will exclude the corroding, surrounding
air.
IBM Corp. mandates this technique in all of their safety grounding schemes.
At least in the labs and on the factory floor.
Next would be the techniques used in dealing with the aluminum wiring found
in many seventies vintage homes. Just simply twisting the conductors
together and wire nutting them is a proven prescription for catastrophic
failure, as in “the house just burned down”.
Reason is that the oxide on the wires acts to insulate them from each other
even while being twisted together. The steel threads of the wire nut cuts
through the oxide and provides the main current path. The wire nut heats
and fails. Google it and see some of the images of burned joints... not
pretty.
The fixes are:
First, a special wire crimping system rented from the Burndy Corp., very
expensive, requires a special tool and technician training, but works very
well.
Next, is the use of a multi-ported sealant filled wiring device which looks
like a three-wide European set-screwed terminal strip. Cheaper, takes less
training to use and are accepted by the industry.
Last, and the one appropriate to the discussion here is the use of an
Alconox like wiring paste. But, it needs to be used differently than just
slathering it onto the joint.
You'll need a green Scotchbrite pad which you will impregnate with the
Alconox (BTW, I'm only using that name as a generic for which ever one is
better suited). Take the now impregnated pad and scour the surfaces to be
joined. The scrubbing action displaces the oxide, the Alconox covers the
now abraded surface and prevents re-oxidation. The wires can now be twisted
together and wire-nutted with good result.
This same technique will work well in making lugged connections to your
aluminum grounding strip.
73,
Stan
-----Original Message-----
From: Mike Fahmie
Sent: Thursday, March 20, 2014 5:37 PM
To: Tower Talk
Subject: Re: [TowerTalk] [RFI] Back of desk grounding buss
I had originally replied to KG2V's query offlist, but after reading other
replies I decided to post my reply to the entire list to share my concerns
about using Aluminum in this application.
-----------
Since I've been bit as the result of an insulating oxide layer on an
Aluminum connection, I wouldn't recommend it for your ground buss. I
use copper, but since it is relatively soft, I don't count on tapped
threads, I use a nut on the backside. If it's heavy enough so that it
is mechanically robust, then it's probably sufficient electrically.
-Mike-
________________________________
From: "n0tt1@juno.com" <n0tt1@juno.com>
To: Charlie@TheGallos.com; rfi@contesting.com; TowerTalk@contesting.com
Sent: Thursday, March 20, 2014 1:51 PM
Subject: Re: [TowerTalk] [RFI] Back of desk grounding buss
FWIW, I just use a 1/8" thick x 3/4" wide aluminum bar
stock from the hardware store. Goes along the back
of the desk (near the top) and is periodly tapped for 8/32 screws
to accept AL/CU rated "spade" lugs that connect to the
various equipment "boxes" with ~1/4" braid. Then the strip is
connected to the station ground. Works for me.
73,
Charlie, N0TT
On Thu, 20 Mar 2014 13:53:19 -0400 Charlie Gallo <Charlie@TheGallos.com>
writes:
Hey Gang,
Sort of antenna, sort of RFI, more safety..
I've decided my back of desk grounding situation needs to be
improved
Now, we all know the standard is a nice, thick copper buss.
Question is, how thick? - this will then be run down to the
panel/ground rod
Is Brass/Aluminum/Copper PIPE a valid alternative? If aluminium,
again, how large (I normally have round up to about 3" diameter
sitting in the shop, as well an AL plate (various sizes, 3/16 thick
up to 6" wide is everyday, and I have angle up to 5" wide legs, 1/4"
thick around, and blocks up to 2" thick and 4" wide hanging around
I intend to silver braze most of my connections, but obviously that
won't work to an Al plate, That I'd drill, tap, and either use
studs with nuts, or screws
Again, we aren't talking the wall pass through here, but the buss at
the back of the desk
--
73 de KG2V - Charles Gallo
Quality Custom Machine-shop work for the radio amateur (sm)
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